The term diphenol is also used for the term bisphenol. Here, the polycarbonate is in particular bisphenol A homopolycarbonate. Bisphenol A is 2,2-bis(4-hydroxyphenyl)propane.
Polycarbonate sheets are known from EP-A 0 110 221 and are supplied for a multitude of applications. They are produced, for example, by extrusion of compositions containing polycarbonate (so-called polycarbonate molding compositions). Multi-layer sheets may be obtained, for example, by coextrusion with polycarbonate molding compositions that contain an increased proportion of UV absorbers.
Polycarbonate molding compositions are understood to mean compositions that contain polycarbonate and that may be formed thermoplastically into molded articles. Coextrusion molding compositions are understood to mean compositions that may be used in the production of multi-layer products, in particular multi-layer sheets, by the coextrusion process.
A problem that recurs frequently when extruding such sheets is the deposition of volatile constituents from the molding compositions on the devices that are used to produce the sheets.
When producing so-called multi-wall sheets (also known as cellular sheets) deposits occur in particular on the so-called calibrator.
Calibrators are known to the person skilled in the art of extrusion technology. They are disclosed, for example, in Kunststoffhandbuch, Volume 3/1, Polycarbonate, Polyacetale, Polyester, Celluloseester, 1992, Carl Hanser Verlag, Munich, Vienna, Page 251 ff, Chapter 3.8.12.2
When producing solid sheets, deposits occur in particular on the rollers, which are a component of the device for the production of solid sheets.
These deposits may lead to surface defects in the sheets.
Volatile constituents, which may be deposited, are for example UV absorbers, mold release agents and other low-molecular constituents of the molding compositions used.
EP-A 0 320 632 discloses co-extruded sheets of polycarbonate, which may contain a UV absorber and a mold lubricant. The disadvantage is that during longer extrusion times, in particular during coextrusion, the surface of the sheets may be adversely affected by vapors emitted by the polycarbonate melt. The increased evaporation of the UV absorber from the polycarbonate melt results in the formation of deposits on the calibrator or the rollers and finally in the formation of defects on the surface of the sheet (e.g. white spots, corrugation etc.).
Polycarbonate abrasion also leads to pulverulent deposits on the co-extruded polycarbonate sheets at the calibrator.
EP-A 0 649 724 discloses a process for the production of multi-layer plastic panels of branched polycarbonates with a weight average molecular weight ({overscore (M)}w) of 27,000 to 29,500 g/mol by co-extrusion of a core layer and at least one top layer containing 1 to 15 wt. % of a UV absorber. If the mold release agents glycerine monostearate, pentaerythritol tetrastearate and mixtures thereof with glycerine monostearate, disclosed in EP-A 0 300 485, are used as the main component in the molding compositions for the production of these plastic panels, the surface of the sheets still deteriorates over time.
Sheets of polycarbonate are also disclosed in EP-A 0 735 074, DE-A 197 27 709, WO 2001/05867, WO 2001 05866, EP-A 0 709 421, EP-A 0 712 878, WO 2001/36511, JP-A 2000254954, JP-A 2000281817, JP-A 2000302893 and JP-A 2000302894. Here, the polycarbonate is produced from diphenols and carbonic acid diaryl esters by transesterification.
JP-A 07053709 discloses a process for the production of polycarbonate by transesterification, wherein the polycarbonate contains less than 0.4 ppm diphenyl carbonate.
WO 00/77073 discloses a process for the production of polycarbonate by transesterification in the melt, wherein the polycarbonate has less than 5 ppm diphenyl carbonate and other low-molecular compounds.